1. Field of the Invention
The present invention relates to the optoelectronic applications of certain MgZnO materials and methods for their manufacture. More particularly, the invention relates to the use of the MgZnO materials in ultraviolet detection devices.
2. Description of the Prior Art
Photodetectors are broadly defined as devices which respond to incident electromagnetic radiation by converting the radiation into electrical energy, thereby enabling measurement of the intensity of the incident radiation. A photodetector typically includes some sort of photoconductive device and external measurement circuitry. Photodetectors have many practical applications. For instance, photodetectors find use in scientific research (such as in scintillation detectors), in manufacturing (such as in devices to detect and prevent spoilage of products by light contamination), and in safety applications (such as in preventing overexposure of workers to certain radiation).
In many applications it is desirable to detect a particular type of light, i.e., a certain range of wavelengths. In such an application, light having a wavelength falling outside the range of wavelengths, which is desired to be detected constitutes “noise” to the photodetector. Noise can cause an erroneous response from the photodetector. Prior art UV photodetectors have the drawback that they typically respond to visible light.
AlxGa1−xN is a compound semiconductor that is ideally suited for devices in the visible and the ultraviolet parts of the spectrum. GaN—AlGaN based solid state ultraviolet (UV) photo detectors, sensitive to 200 nanometer (nm) to 365 nm UV radiation, have been actively sought for applications including solar-blind UV detection and flame sensing. Due to the direct band gap and availability of AlxGa1−xN in the entire alloy composition range (0<x<1), GaN—AlGaN based UV detectors have the advantages of high quantum efficiency, tunability of cut-off wavelengths, and the capability of being fabricated as heterostructures. In recent years, GaN—AlGaN photo conductors and photo diodes of both Schottky and PIN junctions with good performance have been reported. Aluminum gallium nitride has a direct bandgap which is tunable from 3.4 electron volts (or 365 nanometers) at x=0 to 6.2 electron volts (or 200 nanometers) at x=1. This makes the material ideally suited for intrinsic ultraviolet sensors with high responsivities for wavelengths shorter than 365 nanometers and essentially no photosensitivity for longer wavelengths. Such sensors can there for detect ultraviolet emissions from flames in the presence of hot backgrounds (such as infrared emission from the hot bricks in a furnace).
Gallium nitride is a wide, direct bandgap semiconductor which has a broad range of potential applications for optoelectronic and high power/temperature electronic devices. A number of devices have been demonstrated, including high power, short wavelength (blue, violet) light emitting diodes or LED's, ultraviolet photoconductive detectors, ultraviolet Schottky Barrier Detectors, metal-semiconductor field effect transistors or MESFETS, high electron mobility transistors or HEMTS and heterojunction bipolar transistors or HBTs. In the past, several groups of investigators have reported on gallium nitride/aluminum gallium nitride based ultraviolet detectors, including photoconductive, Schottky Barrier, and p-n-junction ultraviolet detectors based on gallium nitride single layers or p-n-junctions. These photo-conductor devices were all of lateral geometry and suffer from several problems. For example, for photoconductors with gains of 1000, the reported bandwidth has only been around 1 kHz, which makes them too slow for many applications. This response speed problem becomes more severe with the addition of aluminum in the active layer.
The lateral Schottky Barrier devices prepared on p-doped gallium nitride were also slow because of the large series resistance of the p-type layer resulting from the lower carrier mobility and in concentration achievable. Further, for the Schottky devices, back illumination through the transparent sapphire substrate side was required. This resulted in poor quantum efficiencies because of light absorption at the gallium nitride-sapphire interface region where a very high dislocation density exists.
The detection of ultraviolet (UV) light during daylight conditions is an important problem for both commercial and military applications. It is difficult to design a very sensitive detector that can be used in broad daylight to detect very low levels of UV radiation. The spectral distribution of radiation from the sun is similar to that of a 6,000 degree blackbody radiator. The solar spectral distribution drops off very sharply below 290 nm due to atmospheric absorption by ozone. As a result, the earth's surface is essentially dark below 290 nm. A solar-blind detector can be defined as a device or apparatus that only responds to wavelengths below about 285 nm. Applications for solar-blind detectors include monitoring lightning events during thunderstorms, detecting ultraviolet laser sources such as excimer lasers or frequency quadrupled Nd:YAG lasers used as LIDAR sources, and ultraviolet telescope detectors for space platforms.
Many prior art approaches have been proposed to achieve solar blind detector performance. One approach, described in U.S. Pat. No. 4,731,881, uses a series of chemical and color glass filters to accomplish UV transmission below 285 nm and a sharp cut off, blocking wavelengths longer than 285 nm. The chemical filters consist of an expensive, single crystal nickel sulfate hexahydrate crystal that has very poor thermal and moisture stability, and an organic dye, Cation X, contained in a polyvinylalcohol film to provide UV bandpass characteristics. This approach uses a relatively expensive UV sensitive photomultiplier tube for detection.
Another approach (described in U.S. Pat. No. 4,731,881) uses a ruby crystal with interference filters coated on the two faces. The input face has a bandpass interference filter that transmits a narrow UV band at approximately 254 nm and rejects all other wavelengths. The output face of the ruby crystal is coated with an interference filter that transmits the ruby fluorescence wavelengths and blocks all other wavelengths. The performance of this device is limited by the band pass and broad band blocking capability of interference filters. A dielectric coating is limited to a rejection of about 105 outside of the bandpass region. An out-of-pass-band rejection of approximately 108 is necessary for true solar blind detection.
Other approaches (described in U.S. Pat. Nos. 4,241,258 and 5,331,168) use UV sensitive phosphor powders as downconverters. Phosphor powders are highly scattering and can result in reduced light collection efficiency.
Visible-blind UV detectors also have great potential in applications such as UV radiometry, flame sensing and missile guidance systems. Currently, photocathodes are the only devices capable of addressing these applications. Unfortunately, these are bulky, difficult to integrate with control electronics and in general, require high operating voltages. Typical prior art devices for achieving visible-blind UV-detection suffer from either excessively low transmission in the UV signal wavelength region or inadequate rejection of visible light.
Other UV detectors are described in U.S. Pat. Nos. 6,104,074; 5,446,286; 5,574,286 and 6,137,123.